Reference is made to document WO 2009102220 A2 which describes the technical field of the present invention, and the challenges with regards to fatigue and tear on subsea components, e.g. at a subsea wellhead. During subsea hydrocarbon extraction, a riser is utilized to establish a conduit between a floating vessel and a subsea wellhead. Due to that the riser in one end is connected to the structure on the seabed and at the other end to a vessel that is under the influence of wind and waves, the riser is experiencing stresses as the vessel moves. The riser is held in tension from the vessel and this will result in bending stresses in the riser as the vessel moves. To minimize these bending stresses, the riser is equipped with a flex joint and or possibly a bend restrictor at the wellhead. A bend restrictor will resist bending and avoid point stresses at the connector, but will not reduce the bending moment, or other forces or stresses resulting in fatigue, as such. An example of a flex joint as used in the industry is shown in U.S. Pat. No. 5,951,061. Such a joint is designed with a certain stiffness to resist bending and, when bending occurs, to force realignment of the riser back to a neutral position.
A constant bending stress in itself will normally not damage the wellhead (or any other weak connection points in the riser) since the connector and the wellhead is designed to withstand these forces. However, the bending may be cyclic, due to vessel movements, and these cycles may result in fatigue problems at the wellhead.
Similarly, a constant tension, compression and/or torsion will normally not damage e.g. the wellhead (or any other desired position) since the connector and the wellhead is designed to withstand these forces. However, the stress, compression and/or torsion may be cyclic, due to vessel movement, and these cyclic movements may result in fatigue problems at the wellhead due to non-constant stresses.
The angular deviation in a flexible joint or connection due to movement of the vessel has a lateral (horizontal) component which gives a bending moment at the wellhead. The movement of the vessel will, in addition to the bending moment created from the lateral part of the angular deviation, also have a component in the length direction (axial) of the riser. There is also possible to have a rotational (torsional) component. All these components will influence the fatigue setting for instance for the wellhead, as these forces will fluctuate leading to variations in the stresses experienced at e.g. the wellhead (if the wellhead is the desired position or one of the desired positions).
Thus, in addition to bending stresses, also compression stress, tension stress and or torsion stress may result in fatigue problems.
The riser has a neutral position, i.e. a position where the stresses and there among the bending moments acting on the riser are low (close to zero). However, due to movements caused by, wind, waves, tension, etc. the riser may move out of its neutral position, wherein some of this movement, at least the movement represented in angular displacement, is allowed by the flexible connection. When this occurs, the riser tends to react by creating a force in the opposite direction compared to the movement out of the neutral position. This opposite direction force is what creates a larger bending moment (and after time: fatigue) on the wellhead (or any other connection/riser part) the most. Thus, to reduce the bending moment, the applicant has solved this issue by, instead of seeking to counteract the movement out of neutral position, rather to apply an additional force which is equal to (or somewhat smaller) than the angular displacement force. The result being that the bending moments experienced at the wellhead is significantly reduced. However, when the forces on the flexible connection acts in another direction, the applied additional force is reduced/shut-off, and the riser is free to move in any direction. Thus, if the riser moves in another direction out of the neutral position than the direction explained above, an additional force may be applied in that direction instead. And, because the riser is moving cyclic, the process is repeated continuously.
Furthermore, the riser is normally connected to heave compensators, which heave compensators (riser tensioning system) make sure that the riser is kept under constant tension, minimizing the loads experience on the subsea wellhead. Under ideal conditions with constant wind, waves and sea currents, it is theoretically possible to keep the riser in constant tension using the heave compensators, and in this way maintaining constant drag forces on the wellhead (or any other desired position along the riser), i.e. to keep the forces from the riser in a neutral position where the sum of forces is at a preferred value. Under such ideal conditions, the wellhead, or any other point along the riser, would experience small, if any, variations in compression/stresses, thereby reducing, or even eliminating, fatigue problems due to compression/tension stresses. However, when used offshore, i.e. when used in real conditions, it has proven that the stress or tension forces experienced in any desired position when using the heave compensators are not always at the preferred value (i.e. referred to as the neutral position), but rather that the forces are higher (downward load) or lower (increased drag) compared to the neutral position/point and fluctuate around the neutral position. In addition, the stresses or forces are not constant due to wind, waves, drift off, etc. Thus, in practice, it has proven that such field conditions result in fatigue problems in said desired position.
An objective of the present invention is to provide a system and method for applying a force on a flexible connection in a riser having even more accurate measurements, i.e. real-time data sets based on real-time measurements.
Furthermore, another objective of the present invention is to provide a solution which compensates for a larger amounts of frictions that may occur in the system.
Another objective of the present invention is to reduce the variations in stresses experienced in a desired position in an offshore production or drilling system.